1. Field of the Invention
The present invention relates to a semiconductor apparatus used mainly as a switching device in, for example, a motor drive device in an inverter, an AC servomotor, an air conditioner, etc., or a power supply device in a vehicle, a welding machine, etc., and more specifically to the improvement of an electrode wiring structure in a semiconductor apparatus applicable as a power semiconductor module.
2. Description of the Related Art
Normally, a semiconductor module can be, for example, a plurality of semiconductor devices (semiconductor chips) connected in parallel to have a larger current capacity, a simple circuit of several types of semiconductor devices, semiconductor devices into which a drive circuit is incorporated, etc.
FIG. 1 is a plan view of an example of a conventional power semiconductor module.
In the semiconductor module shown in FIG. 1, an insulated substrate 2 is mounted on a base plate 1 for fixing. On the insulated substrate 2, a plurality of (four as an example shown in FIG. 1) semiconductor devices (semiconductor chips) 4 are mounted in series through a conductive plate 3. In this example, the semiconductor device 4 is a MOSFET (metal oxide semiconductor field-effect transistor) having a source electrode and a gate electrode on the top side, and a drain electrode on the reverse side.
The conductive plate 3 is electrically connected commonly to the drain electrode of each semiconductor device 4 by mounting the semiconductor device 4 directly on it, thereby functioning as a drain electrode of the entire module. On the insulated substrate 2, a source electrode 5 and a gate electrode 6 of the entire module are respectively mounted along the array of the semiconductor devices 4 and on either side of the conductive plate (drain electrode) 3 on which the semiconductor devices 4 are mounted.
The source electrode 5 is electrically connected commonly to the source electrode of each semiconductor device 4 through a wire (bonding wire) 7, and the gate electrode 6 is electrically connected commonly to the gate electrode of each semiconductor device 4 through a wire (bonding wire) 8. A gate resistor such as a silicon chip resistor, etc. can be provided on the gate electrode 6, and the wire 8 can be connected thereto.
Furthermore, a drain terminal 9 is led outside the module as an external terminal from a portion of the conductive plate (drain electrode) 3, a source terminal 10 is led outside the module as an external terminal from a portion of the source electrode 5, and a gate terminal 11 is led outside the module as an external terminal from a portion of the gate electrode 6.
Although not shown in the attached drawings, the entire module is normally put in a resin package, and the space in the package is filled with gel or epoxy resin, etc. The above mentioned external terminal is drawn in a two-dimensional array in FIG. 1, but it is appropriately bent and exposed on the top or side of the package.
The semiconductor module with the above mentioned configuration has a plurality of semiconductor devices 4 connected in parallel between the drain terminal 9 and the source terminal 10. Therefore, in principle, the main current flowing between the drain terminal 9 and the source terminal 10 can be controlled by applying a control voltage between the gate terminal 11 and the source terminal 10, and simultaneously setting all semiconductor devices 4 ON/OFF.
In the conventional semiconductor module as shown in FIG. 1, restrictions are placed by the gate electrode 6 especially on the wiring pattern from the drain electrode (conductive plate) 3 to the drain terminal 9. That is, the drain terminal 9 is led outside through the path from the end portion of the conductive plate 3 without passing the gate electrode 6.
Therefore, the lengths of the current paths are entirely long when the main current flows from the drain terminal 9 to the source terminal 10 through each semiconductor device 4, and the lengths are uneven depending on the position of each semiconductor device 4. Especially, the current path through the semiconductor device 4 shown in FIG. 1 on the right is considerably longer than the current path through the semiconductor device 4 on the left.
Since the inductance generated in the current path is substantially proportional to the length of the path, the inductance increases correspondingly when the current path is long as described above. As a result, the surge voltage generated when the semiconductor device 4 is turned off rises, thereby possibly destroying the semiconductor device 4.
In addition, when the lengths of current paths are not even, the wiring resistance also becomes uneven depending on the position of each semiconductor device 4. As a result, the current value becomes unbalanced, thereby leading excess current through only a part of the semiconductor devices 4, and also possibly destroying the semiconductor devices 4. Therefore, with the problem of the above mentioned excess current to a part of the semiconductor devices 4 has prevented the maximum current through the module from largely increasing.
Furthermore, with the drain terminal 9 directly connected to the conductive plate 3 to be mounted on the insulated substrate 2 as the semiconductor module as shown in FIG. 1, there can easily be a crack in the joint (the portion encompassed by a circle A indicated by a dot-and-dash line) between the drain terminal 9 and the conductive plate 3 due to the expansion and contraction by the heat from the semiconductor devices 4.
Therefore, to solve the above mentioned problems, the Applicant of the present invention has suggested a semiconductor module having the structure as shown in FIG. 2.
In the semiconductor module shown in FIG. 2, there is the conductive plate 3 mounted on the insulated substrate 2 having a drain electrode 12 on one side, and the source electrode 5 on the other side. On the drain electrode 12, the gate electrode 6 is mounted through an insulating plate (insulating layer) 13.
Furthermore, the drain electrode 12 is connected to the conductive plate 3 through a plurality of wires 14 equally arranged at predetermined distances from one another along the array of the semiconductor devices 4. Thus, the drain electrode 12 is commonly connected to each of the semiconductor devices 4 through the wires 14 and the insulating plate 13.
In addition, two drain terminals 9 are led from the drain electrode 12, and two source terminals 10 are led from the source electrode 5. These drain terminals 9 and source terminals 10 are provided on either side of the conductive plate 3.
With the above mentioned configuration, the drain electrode 12 and the conductive plate 3 are connected through the wires 14 arranged at predetermined distances along the array of the semiconductor devices 4. Therefore, the drain electrode 12 and the insulating plate 13 are equivalent to the structure in which they are directly connected on their sides (the plane along the above mentioned array direction). Therefore, the main current flows substantially straight from the drain electrode 12 to each of the semiconductor devices 4 through the conductive plate 3, and then straight to the source electrode 5. Since the drain terminal 9 and the source terminal 10 are opposite each other, the main current flows substantially straight from the drain terminal 9 to the source terminal 10 through the shortest path.
Thus, since the current path of the main current flows substantially straight from the drain terminal 9 to the source terminal 10, the length of the current path can be the shortest possible. As a result, the inductance can be reduced, and the surge voltage can be suppressed, thereby enhancing the reliability of the entire module.
Furthermore, since the length of the current path can be leveled in the module regardless of the position of each semiconductor device 4, the wiring resistance can be leveled through each current path. As a result, a current does not flow excessively through only a part of the semiconductor devices, thereby leveling the value of the main current, and increasing the maximum current through the entire module.
Furthermore, since the drain electrode 12 is not directly connected to the conductive plate 3, but they are connected indirectly through the wire 14, the conventional crack can be effectively prevented although the semiconductor devices 4 repeat expansion and contraction by their heat.
Thus, with the semiconductor module shown in FIG. 2, the above mentioned problems with the conventional semiconductor module shown in FIG. 1 can be effectively solved.
However, with the configuration in which the drive gate electrode 6 is mounted on the drain electrode 12, the drain electrode 12 requires the space for the gate electrode 6 and the wire 14 for connection as clearly shown in FIG. 3 which is an enlarged sectional view along Bxe2x80x94B shown in FIG. 2. Therefore, the width W1 of the drain electrode 12 is necessarily be large, thereby preventing the realization of a smaller apparatus.
An object of the invention is to provide a smaller semiconductor apparatus with the above mentioned problems with the conventional technology (increasing surge voltage, unbalanced current, cracks, etc.) successfully solved.
To attain the above mentioned object, the present invention has the following configuration.
That is, the semiconductor apparatus according to the present invention includes: a plurality of semiconductor devices mounted in one or more arrays on a substrate; a main current electrode mounted along the array(s) of the semiconductor devices, and commonly connected to each of the plurality of semiconductor devices through the substrate by being connected to the substrate through a plurality of wires; an insulated base mounted on the main current electrode, and covering a joint area between the main current electrode and the wires; and a drive electrode mounted on the base, and commonly connected to each of the plurality of semiconductor devices.
The substrate can be a conductive plate or a conductive layer mounted on an insulated substrate. However, it is obvious that other configurations can be accepted only if a path of the main current flowing from the main current electrode to each of the semiconductor devices can be provided.
The above mentioned main current electrode is a drain electrode or a source electrode when the semiconductor device is, for example, a MOSFET. It also can be a collector electrode or an emitter electrode when the semiconductor device is, for example, a bipolar transistor. The main current electrode is indirectly connected to each semiconductor device through the substrate, that is, connected to the substrate through a wire to form a current path of the main current flowing from the main current electrode to each semiconductor device through the wire and the substrate.
Furthermore, the above mentioned drive electrode is a gate electrode when the semiconductor device is, for example, a MOSFET. It can also be a base electrode when the semiconductor device is, for example, a bipolar transistor. Assuming that the semiconductor device is a MOSFET is used, the drive voltage is normally applied to the gate electrode and the source electrode. Therefore, a drive source electrode can be provided in addition to the source electrode for the main current. In this case, the drive source electrode can be regarded also as the above mentioned drive electrode.
The insulated base does not necessarily indicate an insulating material, but can be accepted only if it insulates the main current electrode from the drive electrode. For example, an insulated base can be obtained by providing an insulating layer on or below a base to insulate the main current electrode from the drive electrode.
According to the present invention, the main current electrode is provided along the array of the semiconductor devices, and the substrate is connected to the main current electrode through a plurality of wires arranged along the array of the semiconductor devices. The plurality of wires are desired to be equally arranged along the array of the semiconductor devices, but are not limited to this arrangement.
With the above mentioned configuration, the main current electrode is actually connected to the substrate indirectly through a plurality of wires. However, since the plurality of wires are arranged along the array of the semiconductor devices, the main current electrode is practically connected to the substrate directly on their sides (planes along the array of the semiconductor devices). Therefore, the main current flows substantially straight from the main current electrode to each semiconductor device through the substrate.
Thus, since the current path of the main current is formed substantially straight from the main current electrode regardless of the position of each semiconductor device, the current path can be the shortest possible, and is leveled. As a result, the inductance can be reduced, and the surge voltage can be suppressed, thereby leveling the main current flowing through each semiconductor device, and increasing the maximum current in the entire semiconductor apparatus (semiconductor module).
Furthermore, the main current electrode is not actually connected directly to the substrate, but is indirectly connected through wires, thereby suppressing the generation of cracks in the joint portions due to the expansion and contraction of the semiconductor devices.
Furthermore, an insulated base is mounted on the main current electrode, and the base covers the connection area between the main current electrode and the wire. The drive electrode is mounted on the base having the above mentioned configuration.
With the above mentioned configuration, the mounting area of the drive electrode on the insulated base can be set close to the semiconductor devices at the position for coverage over the connection area of the wires (that is, such that the mounting area can overlap the connection area of the wires).
As a result of setting the drive electrode closer to the semiconductor devices, the width of the main current electrode can be smaller, thereby realizing a smaller apparatus. Furthermore, as a result of setting the drive electrode closer to the semiconductor devices, the wire connecting the drive electrode to each semiconductor device can be shorter, and the inductance generated in the wire can be reduced.
Various types of structures of the base can be designed. For example, it is desired that the side of the semiconductor devices is beveled and the beveled surface covers the connection area.